Krucinski S, Vesely I, Dokainish M A, Campbell G
Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada.
J Biomech. 1993 Aug;26(8):929-43. doi: 10.1016/0021-9290(93)90055-j.
Recent studies suggest that flexural stresses induced during the opening phase may be responsible for much of the mechanical failures of bioprosthetic heart valves. Sharp leaflet bending is promoted by the mounting of valves on rigid stents that do not mimic the systolic expansion of the natural aortic root. We, therefore, hypothesized that flexural stresses could be significantly reduced by incorporating a flexible or expansile supporting stent into the valve design. Using our own non-linear finite element code (INDAP) and the pre- and post-processor modules of a commercial finite element package (PATRAN), we simulated the opening and closing behaviour a trileaflet bovine pericardial valve. The leaflets of this valve were assumed to be of uniform thickness, with a non-linear elastic behaviour adapted from experimentally obtained bending stiffness data. Our simulations have shown that during maximal systolic valve opening, sharp curvatures are induced in the leaflets near their commissural attachment to the supporting stent. These areas of sharp flexure experience compressive stresses of similar magnitude to the tensile stresses induced in the leaflets during valve closure. By incorporating a stent with posts that pivot about their base, such that a 10% expansion at the commissures is realized, we were able to reduce the compressive commissural stressing from 250 to 150 kPa. This was a reduction of 40%. Conversely, a simple pliable stent with stent posts that deflect inward and outward under load did not achieve a significant reduction of compressive stresses. This numerical analysis, therefore, supports the theory that (i) high flexural and compressive stresses exist at sites of sharp leaflet bending and may promote bioprosthetic valve failure, and (ii) that proper design of the supporting stent can significantly reduce such flexural stresses.
近期研究表明，在开启阶段产生的弯曲应力可能是生物人工心脏瓣膜诸多机械故障的原因。瓣膜安装在无法模拟天然主动脉根部收缩期扩张的刚性支架上，会促使瓣叶急剧弯曲。因此，我们推测，在瓣膜设计中加入柔性或可扩张的支撑支架，可显著降低弯曲应力。我们使用自编的非线性有限元代码（INDAP）以及商业有限元软件包（PATRAN）的前后处理模块，模拟了三叶牛心包瓣膜的开闭行为。假定该瓣膜的瓣叶厚度均匀，其非线性弹性行为根据实验获得的弯曲刚度数据确定。我们的模拟结果显示，在收缩期瓣膜最大开启时，靠近与支撑支架相连的瓣叶连合处会产生急剧的曲率。这些急剧弯曲区域所承受的压应力大小与瓣膜关闭时瓣叶所承受的拉应力相近。通过加入一种带有可绕基部枢转的支柱的支架，使得连合处实现10%的扩张，我们能够将连合处的压缩应力从250千帕降至150千帕。降幅达40%。相反，一种简单的柔性支架，其支柱在负载下向内和向外偏转，并未显著降低压缩应力。因此，该数值分析支持以下理论：（i）在瓣叶急剧弯曲部位存在高弯曲应力和压缩应力，并可能促使生物人工瓣膜失效；（ii）支撑支架的合理设计可显著降低此类弯曲应力。
